[Octave-bug-tracker] [bug #58150] Slow loglog plot with non-default font

[Pantxo Diribarne]

Follow-up Comment #8, bug #58150 (project octave):

We have a font manager that is supposed to cache already loaded fonts. I had
already noticed that this did not work so well (at least no as I would
expect).

I have added the following printf in order to see when a given font face is
erased from the font cache:


diff -r bc6dae9cbe73 libinterp/corefcn/ft-text-renderer.cc
--- a/libinterp/corefcn/ft-text-renderer.cc     Thu Apr 09 15:35:57 2020
-0700
+++ b/libinterp/corefcn/ft-text-renderer.cc     Sat Apr 11 14:25:08 2020
+0200
@@ -412,7 +412,8 @@
       if (face->generic.data)
         {
           ft_key *pkey = reinterpret_cast<ft_key *> (face->generic.data);
-
+          printf ("Destroying font %s\n", pkey->first.c_str ());
+          
           cache.erase (*pkey);
           delete pkey;
           face->generic.data = nullptr;



Here is what I observe:


octave:1> figure ();
octave:2> axes (); hold on
octave:3> plot (1:10)
octave:4> cla
octave:5> loglog (1:10)
Destroying font *:normal:normal
Destroying font *:normal:normal
... (50+ times) ...
Destroying font *:normal:normal


>From this simple session I can notice the following:
* If I don't add the "hold on" statement, the cache is cleared for each axes
reset/redraw. My conclusion is that for most use case this caching is
inefficient: the cache should persist for at least as long as a figure exists
(ideally it would persist for an Octave session).
* after "hold on", a simple "plot" command seems to use cached fonts but log
plots feed/free the cache 50+ times



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